There are several advantages to having a straight riser from the seabed to a surface platform or production vessel that are widely acknowledged in the industry. These include the simplicity of the arrangement, minimisation of pipe and the ability to use a dry tree. This configuration is typically not possible on a floating production vessel or tension legged platforms because a straight riser is unable to absorb the changes in length required to accommodate wave induced or tidal motion. This motion can sometimes be accommodated by heave compensators such as hydraulic rams on the platform and a short flexible interconnect from the top of the riser to the platform. However a direct connection of seabed to platform without or with minimal expensive and complex compensation equipment would be desirable.
Also, it is preferred in the industry to intentionally maintain a riser in tension along its entire length. This is due to the problems which can arise in the event of axial compressive forces being present in regions of the riser, which may lead to issues such as buckling and the like.
The industry has proposed a riser configuration in which the riser extends initially vertically from the seabed, forms a gentle “S”-bend and then terminates into the surface platform or vessel again at a vertical orientation. This configuration is able to absorb substantial vertical motion at the platform or vessel yet uses very little additional pipe. This configuration is defined in the art as a Compliant Vertical Access Riser (CVAR), and heretofore CVAR systems have generally been formed from steel. However the industry has been reluctant to deploy this configuration because it may result in a region of the pipe being in compression which is usually intentionally avoided. Such compression is particularly undesirable in that the geometry of a conventional CVAR includes non-linear portions with extended regions of bending. Such non-linear geometry in combination with compressive axial loading can cause unpredictable behaviour of the riser and may more readily result in yield limits being exceeded.
Furthermore, the combination of dynamic loads and the compressed region of the pipe, and also the typically non-linear geometries, make global analysis and modelling of such riser configurations very challenging as the riser can adopt a large number of shapes. This results in problems predicting the behaviour of such riser configurations under dynamic loads and, in particular, problems in predicting the risk of buckling and the consequential damage that may be incurred under dynamic loads. As such, without confidence in the analysis and modelling of such CVAR systems, the industry is reluctant to deploy them.
Furthermore, conventional CVAR systems may rely upon the attachment of additional weights and buoyancy elements at predetermined points along the riser to provide the required riser shape and to control any compression in the riser. Such additional weights and buoyancy elements add to the complexity and cost of the system and can complicate deployment and recovery of the riser.
Flow-line jumpers may provide compliance in compact space envelopes between two points of attachment, for example, between two fluid ports. Conventional jumpers manufactured from steel or the like typically comprise elbows connected by straight sections for ease of manufacture. These structures fail to minimise the space envelope for a required amount of compliance. Furthermore the presence of sharp 90 degree bends can increase the risk of hydrate build up and restrict hydrate removal operations such as pigging operations.
It is also known to form conduits or jumpers from unbonded flexibles. However, such conduits or jumpers may lose their shape during movement thereof making it difficult to manipulate the conduits or jumpers during deployment and recovery.